1. Field of the Invention
The present invention relates to a seek control system for moving an optical head of an optical storage apparatus to a given position for seek. More particularly, the present invention relates to a seek control system designed to detect the absolute position of an optical head moved by a positioner and to move the optical head to a target position on the basis of the detected absolute position.
In a typical optical disk drive unit (including magneto-optical disk drive unit), illuminating light from a laser diode in an optical head is applied to the surface of a rotating disk-shaped recording medium, i.e., optical disk, thereby recording information. The recorded information is reproduced by detecting changes in properties (quantity of light, plane of polarization, etc.) of the reflected light from the optical disk, which is illuminated with the light from the optical head. The optical disk has information tracks provided over the surface thereof from the inner periphery to the outer periphery of the disk. Accordingly, it is necessary in order to record or reproduce information on or from a desired track to move the optical head for seek in the radial direction of the optical disk. For this purpose, a seek mechanism for moving the optical head radially of the optical disk is needed.
2. Description of the Related Art
A typical optical disk has a user zone containing information tracks and other zones which are used by an optical disk drive unit. The seek operation of the optical head in the user zone is effected by detecting track crossing on the basis of the reflected light from grooves on the optical disk illuminated with the light from the optical head. That is, in the seek operation, the optical head is moved through a distance corresponding to the desired number of tracks by starting counting track crossing detecting pulses at the same time as the optical head begins to move. With this method, the optical head can be accurately positioned with respect to the desired track.
On the other hand, when the optical head is to be moved to a zone other than the user zone, since this zone has no track provided therein and the seek operation must be executed within a short period of time, for example, during the initial processing of the system, the above-described seek method, which is based on the detection of track crossing, is not suitable. Accordingly, it is common practice to employ a seek method wherein the absolute position of the optical head is detected and the optical head is moved to a specified position on the basis of the detected absolute position. With this method, seek can also be effected where no track is provided. In addition, since no track crossing is detected, a high-speed seek operation can be realized.
The above-described method, wherein seek is effected by detecting the absolute position of the optical head, can be effectively carried out by detecting the absolute position of the optical head with an absolute position detector provided in the path of movement of the optical head. Various methods are available for the detection of the absolute position of the optical head. Among them, a method that employs a photo-sensitive detector enables high-speed seek because the absolute position of the optical head can be detected in a non-contact manner and therefore no load is imposed on the seek operation.
Accordingly, there has been proposed a method wherein the absolute position of the optical head is detected with a one-dimensional photo-sensitive detector provided in the path of movement of the optical head. According to the proposed method, the reflected light from the optical disk, which is illuminated with the light from the optical head, is applied to a one-dimensional photo-sensitive detector, and the absolute position of the optical head is detected from a current output corresponding to the illuminated position of the photo-sensitive detector. Then, the drive part for the optical head is controlled on the basis of a difference between the detected absolute position and a target position (for example, see Japanese Patent Application Laid-Open(KOKAI) No.63-281231 (1988)).
With the above-described method, however, the illuminating light from the optical head that is reflected from the optical disk is used as light applied to the photo-sensitive detector. Accordingly, position detection cannot be effected for a seek operation in which the optical head emits no light, e.g., before the adjustment of light emission from the optical head. Therefore, such a seek operation cannot be effected. Further, since the reflected light from the optical disk depends on the quantity of laser light from the optical head and the reflectivity of the medium constituting the optical disk, it is likely that the level of the reflected light will fluctuate., resulting in a lowering in the position detection sensitivity of the photo-sensitive detector. In such a case, accurate seek may not be realized. In addition, the optical path lengthens in order to lead the reflected light from the optical disk to the photo-sensitive detector, so that the light efficiency lowers, and the detection sensitivity of the photo-sensitive detector lowers.